Each Zanpakutō is a manifestation of its owner's soul, and has a unique name which a shinigami is required to learn in order to use it's Shikai and Bankai.

As such each Zanpakutō has unique abilities that match their owner's characteristics and personality, and their names usually hint to what these powers may be.

A Zanpakutō spirit is part of its owner's soul, and often share the same personality traits. At the same time, they are also independent, and thus can be considered a Shinigami's partner in battle. Because they are part of their owner's soul, a Zanpakutō cannot be replaced, though it will slowly regenerate if broken.

[1] As a Shinigami gains power, their Zanpakutō can change form by allowing the inner being to manifest itself to a greater degree in the outside world. The sword and the inner being are synonymous to each other. The spirit of a Zanpakutō dwells within an internal world, most Naturally the wielders soul, which is different for each Shinigami. The Zanpakutō spirit can normally only be seen by its wielder.

No matter what form (or size) a Zanpakutō takes, it is always virtually effortless for its owner to wield because it is a part of its owner's soul.[2] Shinigami with vast spiritual power and who are unable to control it have larger Zanpakutō, as Ichigo Kurosaki did when he first became a Shinigami.

Isshin Kurosaki, Ichigo's father, explains at one point that high-level Shinigami must be capable of maintaining and suppressing the size of their sword. Otherwise, they would end up swinging around swords the size of skyscrapers.[3]

As stated, a Zanpakutō is a being unto itself and tends to be highly temperamental. Its mood is according to how its treated by its Shinigami as well as its overall personality as a part of a Shinigami determines whether it will achieve its true power or not. Ex: Yumichika Ayasegawa calls his Zanpakutō Fuji Kujaku (Wisteria Peacock) even though its true name is Ruri'iro Kujaku (Azure Peacock), knowing that its favorite color is azure and not wisteria purple. He does this so its true power is not seen as his Zanpakutō hates the name, allowing him to pass off his Zanpakutō as a melee-type rather then what it truly is a kido-type (something unacceptable in the 11th Division). Only when its true name is said is its true potential achieved.[4]

A Zanpakutō may find it hard to achieve its shikai or bankai due to its personality which is a reflection of its shinigami's personality.

Communication is key as seen evident with Kenpachi Zaraki, who has little or no communication with his Zanpakutō and therefore doesn't know its name making its true potential not realized. This inability causes his power to damage his Zanpakutō rather then allowing it to truly help him.

Each Zanpakutō has the power to send pluses to the afterlife, purify Hollows, and transform the Zanpakutō itself into more elaborate and powerful forms. Not all Shinigami can hear the name of their Zanpakutō and thus can not achieve the higher tiers of power.